1. Field of the Invention
The invention relates to a process for improving the sulphur yield of a complex for the production of sulphur from an acid gas containing H.sub.2 S, said complex comprising a sulphur plant with a downstream a purification unit.
2. Description of the Related Art
The production of sulphur from an acid gas containing H.sub.2 S is generally carried out in what is conventionally called a sulphur plant, i.e., in an installation into which said acid gas is introduced together with a controlled amount of a gas containing free oxygen, and in which a controlled oxidation of the H.sub.2 S in the acid gas is carried out by means of the oxygen in the gas containing free oxygen in order to produce sulphur, and at the outlet of which a residual gas is withdrawn which contains water vapour and, in an overall amount of between 0.2 and 4% by volume, sulphur compounds including H.sub.2 S, SO.sub.2 and sulphur vapour and/or vesicular sulphur. In most sulphur plants, controlled oxidation of the H.sub.2 S in the acid gas is carried out in order to produce a gaseous effluent containing H.sub.2 S and SO.sub.2 and possibly elemental sulphur, and, after the sulphur which it may contain has been condensed out, said gaseous effluent is brought into contact with a CLAUS catalyst, situated in one or more catalytic oxidation zones operating at temperatures above the dew point of the sulphur contained in the effluent, in order to form a further amount of sulphur by the reaction of H.sub.2 S with SO.sub.2, said sulphur being condensed out. The sulphur compounds present in the residual gas coming from the sulphur plant frequently include at least one of the derivatives COS and CS.sub.2 in addition to H.sub.2 S and SO.sub.2. These derivatives COS and CS.sub.2 may already exist in the charge of acid gas containing H.sub.2 S which is fed into the sulphur plant, and/or may be formed during the oxidation of the H.sub.2 S to sulphur because of the presence of CO.sub.2 and/or hydrocarbons in the acid gas.
The residual gas withdrawn from the sulphur plant is commonly treated in order to reduce its overall content of sulphur compounds to a minimum so that, after incineration, it can be discharged into the atmosphere in accordance with the standards imposed by pollution legislation, and at the same time so that these sulphur compounds are recovered in a form which helps to increase the yield of utilizable products obtained from the gas treated in the sulphur plant.
In various techniques developed for the treatment of a residual gas from a sulphur plant, said residual gas, at a temperature below 160.degree. C., is fed into a purification unit, in which the compounds H.sub.2 S and SO.sub.2 present in the residual gas react with one another to form sulphur, said reaction being carried out at a temperature below the dew point of the sulphur formed, and at the outlet of which a purified residual gas is obtained which is discharged into the atmosphere after it has been incinerated.
In such purification techniques, practically nothing of the organic sulphur compounds COS and CS.sub.2 is retained in the purification unit, and all or part of them ends up in the purified residual gas sent for incineration, which causes considerable sulphur losses and limits the sulphur yield of the complex comprising the sulphur plant and the purification unit to values of about 99%, the sulphur losses which result from the fact that the compounds COS and CS.sub.2 pass through to the incineration stage representing a fraction of the sulphur yield of said complex which can vary from about 0.1 to 0.5%.